Composite and multilayered silver films for joining electrical and mechanical components

ABSTRACT

A silver film for die attachment in the field of microelectronics, wherein the silver film is a multilayer structure comprising a reinforcing silver foil layer between two layers of sinterable particles. Each layer of sinterable particles comprises a mixture of sinterable silver particles and reinforcing particles. The reinforcing particles comprise glass and/or carbon and/or graphite particles. A method for die attachment using a silver film.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a U.S. continuation application of U.S. applicationSer. No. 14/915,067, filed Feb. 26, 2016, which is a U.S. national phaseapplication and claims the benefit of priority under 35 U.S.C. § 371 ofInternational (PCT) Patent Application Serial No. PCT/US2014/053489,titled “COMPOSITE AND MUL TILA YERED SIL VER FILMS FOR JOININGELECTRICAL AND MECHANICAL COMPONENTS” and filed on Aug. 29, 2014, whichin turn claims priority to U.S. Provisional Patent Application Ser. No.61/871,435 filed on Aug. 29, 2013, the contents of all of which arehereby incorporated herein by reference in their entireties for allpurposes.

FIELD OF THE DISCLOSURE

The present disclosure is directed to methods of joining electrical ormechanical components, and more particularly to methods of attachingelectronic components and associated devices onto circuit boards.

BACKGROUND

Sintering has emerged as an alternative technology to conventionalsoldering. Sintering typically involves high temperature and highpressure processing to attach various components of a printed circuitboard assembly.

SUMMARY

In accordance with one or more aspects of the present disclosure, asilver film may comprise a mixture of silver and reinforcing (modifying)particles. Aspects of the silver film further may include providing amixture comprising a sinterable silver particles layer, and reinforcingparticles. The silver film further may comprise a support filmconfigured to support the sinterable silver particles layer and thereinforcing particles. The reinforcing (modifying) particles can bemetallic or non-metallic. The reinforcing (modifying) particles can beof various shapes and sizes, including spheres, granular, flakes,fibers, flowers and nanowires. The size of the reinforcing (modifying)particles can range from 2 nm to 10 μm. The reinforcing (modifying)particles that can be used to modify the properties of a sintered silverjoint are copper, aluminum, glass, carbon, graphite and others. Aconcentration of the reinforcing (modifying) particles can range from0.1 wt % by weight to 50 wt % by weight. The silver film may be amultilayer structure composed of at least two layers but can be threelayers, including a reinforcing foil layer. The reinforcing foil layercan be silver, copper, gold or any other metal or any alloy. Thereinforcing layer can be a metalized polymer or a ceramic foil. Thereinforcing metal foil layer can be composite or clad structures withdifferent layers of different metals and alloys. The reinforcing metalfoil layer can be solid, perforated or in the form of mesh.

One or more other aspects may involve lamination processes using thedisclosed films. Still further aspects may involve die attachmentmethods using the disclosed films.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments are discussed in detail below. Embodimentsdisclosed herein may be combined with other embodiments in any mannerconsistent with at least one of the principles disclosed herein, andreferences to “an embodiment,” “some embodiments,” “an alternateembodiment,” “various embodiments,” “one embodiment” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide illustration and afurther understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of the invention. Where technicalfeatures in the figures, detailed description or any claim are followedby references signs, the reference signs have been included for the solepurpose of increasing the intelligibility of the figures anddescription. In the figures, each identical or nearly identicalcomponent that is illustrated in various figures is represented by alike numeral. For purposes of clarity, not every component may belabeled in every figure. In the figures:

FIG. 1 presents a schematic of a joint in accordance with one or moreembodiments;

FIG. 2 presents a schematic of a composite silver film in accordancewith one or more embodiments;

FIGS. 3 and 4 present schematics of composite silver films in accordancewith one or more embodiments;

FIGS. 5A-5E present examples of metal foils in accordance with one ormore embodiments;

FIG. 6 presents a schematic of a die attach structure in accordance withor more embodiments; and

FIGS. 7-11B present data discussed in the accompanying Examples.

DETAILED DESCRIPTION

Disclosed herein are embodiments of a sintered silver film composition,which improves the strength and elasticity of sintered silver joints.The improvement in strength and elasticity is achieved by addition ofreinforcing particles and/or a solid metal layer to the sinterablematrix.

Sintered silver die attach films combine unique physical properties ofnanosilver powder and innovative chemical formulations into productswhich allow joining various electronic devices to produce extremelyreliable high thermal and electrical conductivity interfaces. Such filmsinclude those disclosed in co-owned, co-pending U.S. Patent ApplicationPublication No. 2012/0114927 A1 (“SINTERING MATERIALS AND ATTACHMENTMETHODS USING SAME”) to Khaselev et al. which is hereby incorporatedherein by reference in its entirety for all purposes. Sintered silverdie attach films are uniquely positioned to fit into existingmanufacturing equipment and processes to enable high volumemanufacturing. The technology covers a wide variety of devices andapplications from large area thyristors and power modules for electricaland automotive equipment to miniature discrete components for mobiletechnology and LED lighting. The technology improves the performance ofexisting power devices and increases their reliability five to ten timescompared to traditional joining techniques. The films enable use of thenew high temperature SiC and GaN devices with efficiency unattainablewith the existing technologies.

Silver die attach films are the joining material which attaches anelectronic device to a passive substrate or to another device. Silverfilm is applied to the back side of the die, wafer or substrate. Thenthe die, wafer or substrate is placed on a preheated substrate with aforce sufficient to densify the film and establish an intimateconnection between the material and the connected parts. Under theapplied heat and pressure the film sinters and connects die tosubstrate, wafer to wafer, substrate to substrate. The resulting jointbetween the die and the substrate is metallic silver with the structureand properties as shown in FIG. 1.

For certain applications, modification of mechanical properties of thesintered silver joint is desirable. For example, higher elasticity willimprove joint reliability for devices in which the CTE of connectedparts is very different. Also, higher strength of the sintered silverlayer will improve performance of the devices operating under highthermal or mechanical stress.

In accordance with one or more embodiments, a silver film may be amixture of silver and reinforcing (modifying) particles, asschematically shown in FIG. 2.

The reinforcing (modifying) particles can be metallic or non-metallic.The particles can be of various shapes such as spheres (round),granular, flakes, flowers, fibers and/or nanowire. The size of theparticles can range, for example, from about 2 nm to about 10 μm andabove. Examples of particles that can be used to modify the propertiesof sintered silver joints are copper, aluminum, glass, carbon, graphiteand others. The concentration of these particles can range from about0.1 wt % to about 50 wt % in various embodiments.

In accordance with one or more other embodiments, the silver film is amultilayer structure composed of at least two layers, but can be threeor more layers: sinterable silver particles layer/metal foil layer orsinterable silver particles/metal foil/sinterable silver particles layeras schematically shown in FIGS. 3 and 4, respectively. Silver sinterablelayers 1 and 2 can have the same composition and structure or can bedifferent.

The reinforcing metal foil layer can be silver, copper, gold or anyother metal or any alloy, such as metalized polymer and/or ceramic foil.The foils can be composite or clad structures with different layers ofdifferent metals and alloys mentioned previously. The metal foils can besolid, perforated or in the form of mesh. Examples are shown in FIGS.5A-5E.

The metal foils can be plated such as with Ag or Au for bettersintering. The foil thickness can range from less than about 1 μm toabout 300 μm and more. There is no real practical limit to thethickness. Any thickness that is considered a foil is suitable. Thefilms can be supported by a thin plastic layer or stand alone.

The composite sinter silver films may be used in manufacturing ofelectronic and other devices in the same manner as non composite sintersilver films. The methods of using such films are described in detail inU.S. Patent Application Publication No. 2012/0114927 A1, which is herebyincorporated herein by reference in its entirety for all purposes. Suchmethods are applicable to the modified composite films discussed hereinand may include lamination and die attach processes. A resultingstructure is schematically shown in FIG. 6.

The silver sinterable films are composed of a matrix of silvernanoparticles and modifying material in the form of particles or films.The modifying material can be metallic and/or non-metallic powder ofvarious shapes and/or solid metal layer. The composite silver films aredesigned to improve strength and elasticity of sintered silver joints.The composite films can be used for the joining of various parts forelectronics manufacturing and other applications. The methods to use thesilver sintered films are described in U.S. Patent ApplicationPublication No. 2012/0114927 A1 incorporated by reference above.

As used herein, reflow soldering refers to a process in which solderpaste is printed or dispensed, or a solder preform is placed, or both,on a surface of a circuit board, components are placed in or near thedeposited solder, and the assembly is heated to a temperature sufficientto reflow the solder.

The methods and compositions described herein may be used inapplications including, but not limited to, fabrication of powermodules, power discrete devices, bipolar, power devices, thermoelectriccoolers, energy harvesting devices, LED and MEMS assembly, stackedmicroprocessor and memory devices.

The function and advantages of these and other embodiments will be morefully understood from the following examples. The examples are intendedto be illustrative in nature and are not to be considered as limitingthe scope of the embodiments discussed herein.

EXAMPLES Example 1

The silver films are made according to the procedure and formulationdescribed in U.S. Patent Application Publication No. 2012/0114927 A1incorporated herein by reference. The silver film composition is 98% byweight of silver nanoparticles and 2% by weight of organic binder (FilmA). The performance of this film is compared to the one containinggraphite particles. The composition of composite film is 97% by weightof silver nanoparticles, 1% by weight of graphite spherical particles of˜2-3 μm diameters, and 2% by weight of organic binder. Silicon dies 5×5mm were attached to 3 mm thick copper substrates. The attachment wasdone at 10 MPa pressure, 250 C for 90 s.

The die attach samples were subjected to liquid-to-liquid thermal shocktest with temperature swing from −40° C. to 125° C. with 3 min hold ateach temperature. Associated data is presented in FIG. 7. After 600cycles the dies attached with silver film cracked and partially detachedfrom the substrates as shown in the optical images. The parts attachedwith the composite film did not crack and appeared strongly attached tothe substrates. Additional examination of these samples with scanningacoustic microscope showed no degradation of the composite silver jointdue to thermal shock.

Example 2

The multilayer composite film is made by placing solid silver foilbetween two layers of sinterable nanosilver particles. The structure ispressed together and heated to 130° C. for 1-2 minutes. Due to anapplication of pressure and heat, the silver particles layers adhere tosilver foil. The resulting composite multilayer film can be easilyhandled manually or with pick-and-place equipment. The multilayer filmcan be cut to a required die size to form free standing foils the samesize as the dies to be attached. The multilayer film can also belaminated to the back side of the wafer which can be further diced tothe individual dies as described in U.S. Patent Application PublicationNo. 2012/0114927 A1 hereby incorporated by reference.

The multilayer composite silver film is used to attach Si dies 11×11 mmto copper finished DBC substrates. The attachment was done at 10 MPa,250° C. for 90 seconds. The strength of the die attachment was tested bybending the samples across 10 mm mandrel. As shown in FIG. 8, the diesuniformly cracked without any de-lamination from the substrateindicating strong attachment.

The samples are cross sectioned and examined with a scanning electronmicroscope (SEM). Low magnification images, as shown in FIG. 9,demonstrated uniform attachment and bond line thickness across theentire die.

The high magnification SEM images as shown in FIG. 10 revealed detailsof the sintered joint. The top and the bottom sintered silver layersshowed dense structure with good connection to the die, substrate andintermediate layer of silver foil. The thickness of the top sinteredlayer was ˜10 μm and the bottom one 6 μm. The silver foil thickness was9 μm.

The die attach samples are subjected to liquid-to-liquid thermal shockwith a temperature swing from −55° C. to 175° C. with 3 minutes hold atthe temperature. The CSAM images shown in FIGS. 11A-11B reveal no jointdegradation indication strong bonding after 500 thermal shock cycles.

It is to be appreciated that embodiments of the materials and methodsdiscussed herein are not limited in application to the details ofconstruction and the arrangement set forth herein. The materials andmethods are capable of implementation in other embodiments and of beingpracticed or of being carried out in various ways. Examples of specificimplementations are provided herein for illustrative purposes only andare not intended to be limiting. In particular, acts, elements andfeatures discussed in connection with any one or more embodiments arenot intended to be excluded from a similar role in any other embodiment.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use herein of“including,” “comprising,” “having,” “containing,” “involving,” andvariations thereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items.

Having described above several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure and are intended to be within the scope of the invention.Accordingly, the foregoing description and drawings are by way ofexample only.

The invention claimed is:
 1. A silver film, wherein the silver film is amultilayer structure comprising a reinforcing silver foil layer betweentwo layers of sinterable particles wherein each layer of sinterableparticles comprises a mixture of sinterable silver particles andreinforcing particles, the reinforcing particles comprising glass and/orcarbon and/or graphite particles, wherein the silver film is configuredfor die attachment by application of heat and pressure to sinter thesilver film, and wherein the reinforcing silver foil layer is ametalized polymer or a ceramic foil comprising silver cladding.
 2. Thesilver film of claim 1, wherein the reinforcing particles comprise saidglass particles.
 3. The silver film of claim 1, wherein the reinforcingparticles consist of said glass particles.
 4. The silver film of claim1, wherein the reinforcing particles comprise said carbon particles. 5.The silver film of claim 1, wherein the reinforcing particles consist ofsaid carbon particles.
 6. The silver film of claim 1 wherein thereinforcing particles comprise said graphite particles.
 7. The silverfilm of claim 1, wherein the reinforcing particles consist of saidgraphite particles.
 8. The silver film of claim 1, wherein thereinforcing silver foil layer is configured to support the sinterablesilver particles layer and the reinforcing particles.
 9. The silver filmof claim 1, wherein the reinforcing particles are selected from thegroup consisting of spheres, flakes, fibers, flowers, nanowire, andcombinations thereof.
 10. The silver film of claim 1, wherein thereinforcing particles have a particle size between 2 nm and 10 μm. 11.The silver film of claim 1, wherein the mixture of sinterable silverparticles and reinforcing particles comprises between 0.1 wt % by weightand 50 wt % by weight of the reinforcing particles.
 12. The silver filmof claim 1, wherein the reinforcing silver foil layer is composite or aclad structure with different layers of different metals and alloys. 13.The silver film of claim 12, wherein the reinforcing silver foil layeris perforated or in the form of mesh.
 14. The silver film of claim 1,wherein the two layers of sinterable particles are in direct contactwith the reinforcing silver foil layer.
 15. The silver film of claim 1,wherein the silver film consists of the reinforcing silver foil layerand the two layers of sinterable particles.
 16. The silver film of claim15, wherein the reinforcing silver foil layer is sandwiched between andin direct contact with the two layers of sinterable particles.
 17. Thesilver film of claim 1, wherein the silver film consists of thereinforcing silver foil layer, the two layers of sinterable particles,and a support layer.
 18. The silver film of claim 17 wherein thereinforcing silver layer is sandwiched between and in direct contactwith the two layers of sinterable particles and the support layer isattached to one of the layers of sinterable particles.
 19. A method ofattaching a die to a substrate comprising: applying the silver film ofclaim 12 to a die; and placing the die on a substrate with applied heatand pressure to sinter the film and connect the die to the substrate.